"Creating mailbox file: File exists" is more a warning - the easy fix is just to delete the file.
To fully clean out a user requires "userdel -r <username>". This deletes the users home directory (and all the file), and deletes the users mail file. Without the -r, it only removes the login name and entries from the groups file. Frequently, this is preferred because somebody usually has to go through the files and decide what to keep, or to discard. That includes the mail file...
So leaving these around generates warnings about things that exist, that possibly shouldn't.